There is considerable interest in phase shift masks as a route to extending resolution, contrast and depth focus of lithographic tools beyond what is achievable with the normal binary mask technology. Among the several phase shifting schemes, the (embedded) attenuating phase shift masks proposed by Burn J. Lin, Solid State Technology, January issue, page 43 (1992), the teaching of which is incorporated herein by reference, is gaining wider acceptance because of its ease of fabrication and the associated cost savings.
Several technical variations of attenuated phase shift masks have been proposed. In a first variation, the substrate is provided with a slightly transparent layer, e.g. a very thin chrome layer, coupled with etching into the quartz substrate to produce the desired phase shift. This method requires a high degree of control of both layer deposition and the etch process. In other variations, a phase shift mask is provided by applying one or more layers with phase shifting and attenuating properties on the substrate. There have been proposed single layer solutions in which one layer provides the 180° phase shift as well as the attenuation of the incident light. Such single layer solutions are e.g. described in U.S. Pat No. 5,942,356, U.S. Pat. No. 5,635,315, U.S. Pat. No. 6,503,644, U.S. Pat. No. 5,939,225, U.S. Pat. No. 5,477,058 and U.S. Ser. No. 2002/0119378 A1. Single layer solutions had been chosen due to their simple structure and, therefore, their easier preparation. However, single layer solutions are limited in view of an independent tuneability of transmission and phase shift. In particular, high transmission attenuated phase shift mask blanks for an exposure wavelength of 193 nm and phase shift mask blanks for an exposure wavelength of 157 cannot be achieved. Besides single layer solutions, bilayer and multilayer attenuating phase shift mask blanks have also been described. Multilayers have been described e.g. in U.S. Pat. No. 5,897,977 and U.S. Pat. No. 6,274,280. U.S. Pat. No. 5,897,977 relates to embedded attenuating phase shift mask blanks (EAPSM) for wavelength of less than 400 nm comprising distinct alternating layers of an optically transparent material such as a metal oxide, metal nitride oralkaline earth fluoride and layers of an optically absorbing material, such as an elemental metal, metal oxide or metal nitride. U.S. Pat. No. 6,274,280 describes EAPSM for exposure wavelengths of less than 200 nm comprising distinct alternating contiguous layers of an optically transparent material consisting essentially of an oxide selected from the group consisting of oxides of Al and Si and layers of an optically absorbing material consisting essentially of a nitride selected from the group consisting of nitrides of Al and Si. Single and multilayer solutions for phase shift mask blanks for exposure wavelength of less than 160 nm are also described in U.S. Pat. No. 6,395,433. The phase shift system comprises at least one material with at least silicon, silicon oxide or silicon nitride and absorbing metal oxides or nitrides to decrease the transmission properties of the phase shift mask blank. This document stresses that each of the layers of the multilayer should be sufficiently thin to result in a multilayer acting as a pseudo single layer. Multilayer solutions are less preferred for exposure wavelengths less than 200 nm since defects cannot be repaired.
Several publications mention bilayer phase shift mask blanks: JP 04-068352 A relates to a phase shift mask having a high accuracy that can be easily inspected and corrected. U.S. Ser. No. 2002/0122991 A1 describes single layer and bilayer halftone phase shift mask blanks comprising a phase shift layer constituted of silicon, oxygen and nitrogen. Optionally, an etch stop layer provided between the substrate and the phase shift layer. The transmission of the phase shift layer is adjusted by changing the ratio of oxygen and nitrogen in the phase shift layer. According to this document, if the range of nitrogen in the phase shifter layer is less than 5 atomic % or the range of oxygen exceeds 60 atomic %, the transmittance of the film is too high and the function of the halftone phase shift layer is lost. U.S. Pat. No. 5,482,799 relates to a bilayer phase shift mask blank wherein the phase shift layer includes a monolayer formed of an approximately homogeneous material and a transmitting film whose transmittance is less dependent on the wavelength when used in combination with said monolayer film. Such a dependency on the optical properties of one layer of a phase shift system from another layer of a phase shift system is disadvantageous for a phase shift system in that the phase shift and transmittance can not be tuned independently. U.S. Pat. No. 6,458,496 describes phase shift mask blank having a bilayer phase shift system. The mask blanks have an improved etching selective ratio to the substrate. TaSiO is described as the preferred material for imposing a phase shift to the phase shift mask blank.
None of the cited documents related to bilayer solutions addresses the problem of defects in the deposited layers or the uniformity of the layers in phase shift mask blanks for exposure wavelengths for 200 nm or less.
With the increasing requirements for the decreasing feature sizes of photomasks, substantially defect free photomask blanks are becoming more and more important. Defects on the photomask blank may lead to defects such as pinholes in the photomask which results in defects in the IC devices. The task to avoid defects on mask blanks is becoming more challenging due to the decrease of the feature sizes. For example, for the 65 and 45 nm nodes (i.e., feature sizes of 65 nm and 45 nm, respectively, on the wafer), a photomask is patterned with structures having a feature size of 100 nm and thus must be free from surface defects having a particle size of more than 0.5 μm.
It is therefore an object of the present invention to provide novel phase shift mask blanks for exposure wavelengths of 200 nm or less that combine the possibility of an easy and stable production with the necessary optical properties, chemical stability as well as a defect free surface and uniformly deposited layers.